Cauliflower mosaic virus (CaMV) is a double-stranded DNA plant virus. It contains two promoters responsible for the production of transcripts of 35S and 19S in size in infected plants (1). The 35S promoter has been studied in considerable detail (2, 3). This promoter is not only active in isolated protoplasts of monocots and dicots (4) but is also expressed in all organs of transgenic petunia and tobacco in the absence of any viral protein (2, 5). The high activity of the 35S promoter and its apparent constitutive expression have made it an attractive model system to investigate cis regulatory elements for plant gene transcription.
Several 5' deletion mutants of the 35S promoter have been analyzed in transformed tobacco calli as well as in transgenic tobacco plants (2, 6). Whereas a promoter containing only 46 bp of 5' sequence was sufficient for accurate transcription initiation, sequences between -46 and -105 can significantly increase the expression level. In a recent detailed analysis of the 35S promoter (-343 to +9) in vivo, it was reported that an internal deletion of the -107 to -46 region leads to a 60% decrease in transcription activity (6). More dramatically, the activity of an upstream element (-343 to -208) is only detectable when fused to the 35S promoter deleted to -90 but not to the -46. Since the -90 promoter element has little activity in vivo except in root (7), these data suggest the existence of an element in the -90 to -46 region of the 35S promoter, that modulates the activity of elements upstream. Results from transient assays of deletion derivatives of the 35S promoter have also indicated the presence of an element 3' of -90 that is required for expression of upstream elements (3). Because of the proximity of this region to the TATA box, it has been suggested that the relevant sequences are among the three CAAT box-like motifs located between -90 and -46 (2, 3).